Ceramic compositions comprising lead, zirconate, and titanate (i.e, so-called PZT compositions) have numerous uses in electrical devices. For instance, ferroelectric ceramic compositions comprising PZT can be used as piezoelectric transducers, electroactuators, capacitors, and electro-optic devices. Additionally, ferroelectric PZT materials can be used as thin film deposition targets in forming portions of non-volatile ferroelectric random access memories (FeRAMs), pyroelectric devices and micro-electromechanical systems (MEMS) by physical vapor deposition.
PZT can be modified for particular applications. For instance, lanthanum modified PZT (referred to as PLZT) is reported to have exceptional properties for utilization in FeRAMs.
PZT ceramics have a number of advantages for utilization in electrical devices. For instance, PZT ceramics can have a high electromechanical coupling coefficient, which can render them particularly useful as piezoelectric transducers. Additionally, PZT ceramics can have a high pyroelectric coefficient, which can make them particularly suitable as infrared detectors. Further, PZT materials can have a high dielectric constant, which can render them particularly useful as dielectric materials in capacitor constructions. Also, PZT ceramics can have superior electrooptic properties, which can render them suitable as electro-optic switches.
A difficulty in utilizing PZT ceramics is in fabrication of suitable PZT materials. Specifically, a number of the PZT applications described above utilize relatively large-dimension PZT ceramics. It can be difficult to control processing conditions to insure high density and good reproducibility throughout a large PZT ceramic. An exemplary application wherein a relatively large PZT ceramic is utilized is physical vapor deposition. Specifically, a PZT ceramic can be utilized as a sputtering target for physical vapor deposition during formation of small-scale circuits, such as, for example, FeRAMs. It is desired that the target be comprised of homogeneous grains having dimensions of 1 micrometer or less (with smaller grain sizes being more desirable than larger grain sizes), have a density approaching maximum theoretical density, and be a single phase perovskite structure. Homogeneity and fine grain size of the target are desired in order that the target can be utilized to deposit films with appropriate chemical composition, uniformity, and preferred orientation at a rapid deposition rate.
A conventional method of forming PZT materials is to densify a mixture comprising lead, zirconium, and titanium. The lead, zirconium and titanium can be in the form of, for example, PbZrO.sub.3, PbTiO.sub.3, and/or Pb(Zr, Ti)O.sub.3. One method of densifying such mixture is through sintering, another method is through hot-pressing, and yet another is through sinter-forging.
A typical sintering method is pressureless (i.e., occurs at atmospheric pressure, and for purposes of interpreting the claims, sintering is to be assumed to be pressureless unless stated otherwise), and is as follows. First, a mixture comprising lead, zirconium and titanium is cold-pressed into a so-called green compact. The mixture can further comprise a binder, such as, for example, polyvinyl acetate. The term "cold-pressing" refers to pressing occurring at or below about 30.degree. C., and typically at a pressure of about 10,000 pounds/in.sup.2. Once formed, the cold-pressed pellet is subjected to a temperature of at least 1100.degree. C., and typically from 1200.degree. C. to 1400.degree. C., to sinter the pellet. The high temperatures of the sintering process utilize a large amount of heat energy. Such heat energy can deteriorate and erode the interior of a sintering furnace. Accordingly, maintenance costs of sintering furnaces can become expensive. Further, the high temperatures utilized for the sintering process can result in the relatively volatile material PbO being released from the ceramic. The release of PbO removes lead. The loss of lead can alter a composition of the ceramic, and can complicate reproduction of PZT ceramics in sequential sintering processes.
A method which has been utilized to compensate for the loss of lead is to add additional lead to a PZT composition prior to sintering. However, the additional lead can cause its own problems in the form of inhomogeneous lead distribution, and difficulty in controlling lead concentration and lead-site vacancies within a PZT ceramic formed from a material comprising excess lead.
As mentioned above, hot-pressing can be utilized instead of sintering for densifying a PZT. Hot-pressing typically comprises placing a mixture of lead, zirconium, and titanium in a press and compressing the mixture to a pressure of less than 10,000 pounds/in.sup.2 and typically less than 6,000 pounds/in.sup.2. During the pressing, a temperature of the material is typically maintained at more than 700.degree. C. An advantage of utilizing hot-pressing instead of sintering can be that hot-pressing frequently is done at lower temperatures than sintering. The lower temperatures of hot-pressing can avoid some of the above-discussed problems associated with sintering. However, a difficulty with hot-pressing is that relatively large equipment is used to press even a small amount of ceramic material. The expenses associated with forming large-scale hot-pressing facilities for pressing large amounts of PZT materials reduces the economic feasibility of hot-pressing processes. Further, the throughput in production of large-scale PZT ceramic materials using hot-pressing is lower than can be accomplished with sintering. Accordingly, sintering can be more attractive than hot-pressing for commercial-scale PZT production.
The last of the above-identified methods of densifying PZT materials, sinter-forging, involves subjecting a green compact (a green compact is referred to above in describing the sinter process) to hot-pressing. Sinter-forging methods can be difficult to commercialize for reasons similar to those discussed above regarding hot-pressing methods.
Because of the commercialization potential of sintering processes, several methods have been utilized in an attempt to improve sintering processes. Among such methods is the addition of sintering aids, such as, for example, metal oxides and fluoride compounds to PZT ceramic compositions. Such additions can reduce sintering temperatures, and thus lower manufacturing costs, while enabling reasonable control of the lead content in resulting PZT ceramics. In particular applications, utilization of additives has been shown to reduce sintering temperatures down to 950.degree. C. for PZT compositions which contain lower valence substituents (e.g., Fe, Mn). However, additives have not been found which can successfully reduce a sintering temperature below 1050.degree. C. for bulk PZT compositions comprising lanthanum. Further, no process (either sintering, or hot-pressing) has produced a PZT material with a density greater than or equal to 95% of a theoretical maximum density of the material (about 7.9 gm/cm.sup.3) while keeping a predominate grain size within the PZT material to less than 500 nanometers. It would be desirable to develop dense PZT materials with small grain sizes for many applications, including, for example, physical vapor deposition target applications.